1. Field of the Invention
The present invention relates to a magnetic tunnel junction structure with perpendicular magnetic anisotropy (PMA) free layers.
2. Background of the Invention
Magnetic random access memories (MRAMs) currently developed for practical applications utilizes a magnetic tunnel junction (MTJ) element that forms a ferromagnetic tunnel junction as a magneto-resistive device. The MTJ device includes tri-layer consisting of two magnetic layers separated by a non-magnetic layer, and current flows by tunneling through the non-magnetic layer (tunnel barrier). As another structure of MTJ device, a so-called spin-valve structure is known in which an antiferrormagnetic layer is assigned close to a magnetic layer at one side for the purpose of improving magnetic field sensitivity and a magnetization direction is fixed.
In case of a conventional MRAM, the magnetization of a ferromagnetic material constituting a unit cell is typically controlled by magnetic field. By contrast, it is possible to realize a current-induced magneto-resistive device in which the magnetization of a ferromagnetic material can be changed by applying a spin-polarized current. The current-induced magneto-resistive device is referred to as a device in which the magnetization direction can be controlled by applying a current to the magnetic layer thereof, differently from a typical magneto-resistive device in which the magnetization direction of a magnetic layer is controlled by applying a magnetic field.
The method of reading information in a current-induced magneto-resistive device is similar to the case of a conventional magnetic field-induced MTJ or a giant magneto-resistance (GMR) device. The device has a low resistance value if the magnetization orientation of a free magnetic layer and that of a fixed magnetic layer are in parallel configuration, and a high resistance value if they are in anti-parallel configuration, which correspond to digital information “0” and “1,” respectively.
The major difference between a current-induced magneto-resistive device and a magnetic field-induced MTJ or GMR device lies in the method of recording information. In case where the magnetization direction of a fixed magnetic layer is opposite to that of a free magnetic layer and electrons are flowing from the fixed magnetic layer to the free magnetic layer, current spin-polarized by the fixed magnetic layer are injected into the free magnetic layer. In this case, an amount of spin angular momentum of electrons is transferred into the free magnetic layer, and if an amount of the angular momentum exceeds a threshold level, then the magnetization direction of the free magnetic layer is reversed in the direction parallel to that of the fixed magnetic layer. In contrast, in case where the magnetization direction of the fixed magnetic layer is the same as that of the free magnetic layer, and electrons are flowing from the free magnetic layer to the fixed magnetic layer, electrons having a spin of the same direction to that of the fixed magnetic layer are injected into and passed through the fixed magnetic layer, and some of electrons having a spin of the opposite direction to the fixed magnetic layer are reflected at the interface of the non-magnetic layer and fixed magnetic layer and returned to the free magnetic layer. Those electrons exerts a torque opposite to the spin direction of the free magnetic layer and reverses the magnetization direction of the free magnetic layer, thereby allowing the direction of the two magnetic layers to be anti-parallel.
In addition, the current-induced magneto-resistive device may also be applicable to a current oscillator at a frequency band of microwave applications by using the same principle.
Such a current-induced magneto-resistive device has a typical problem in that an amount of current required for reversing the magnetization direction is large. If the current is large, then it is difficult to fabricate a drive circuit for controlling a device. In general, one transistor is connected to each device to constitute a memory by using a current-induced magneto-resistive device. The amount of current is limited by the size of a transistor, and thus the size of a transistor should become larger to allow more current to flow therethrough.
Such a problem may cause an increase of the power consumption in driving a device and also gives rise to difficulties in the integration of a device. As a consequence, it is required to reduce the amount of current necessary to reverse the magnetization direction in a current-induced magneto-resistive device.
Furthermore, as the device becomes smaller in size, it becomes possible to switch a magnetization direction by thermal fluctuation, which may cause recorded information to be lost unexpectedly. To avoid this problem, magnetic anisotropy energy (KV, where K denotes an anisotropic energy density, V denotes a volume of a device) should be kept higher than thermal energy (kBT, where kB denotes Boltzmann constant, and T denotes temperature).